A large variety of optical deflectors having a resonance-oscillated mirror have been proposed. The resonance type optical deflector has the following features, as compared with a conventional optical-scanning optical system using a rotary polygonal mirror such as a polygon mirror: that is, the size of the device can be made very small and the power consumption can be kept low. Particularly, an optical deflector comprising Si monocrystal, which is produced through the semiconductor process, has theoretically no metal fatigue and thus the durability thereof is very good.
On the other hand, in the resonance type deflector, since the displacement angle of the mirror changes theoretically sinusoidally, the angular speed of deflected light is not constant. The following techniques have been proposed to correct this characteristic (see U.S. Pat. No. 4,859,846, U.S. Pat. No. 5,047,630 and U.S. Pat. No. 7,271,943).
In U.S. Pat. No. 4,859,846 and U.S. Pat. No. 7,271,943, a resonance type deflector having an oscillation mode based on a fundamental frequency and a frequency three-fold the fundamental frequency, is used to achieve the driving in which the displacement angle of the mirror changes like a chopping wave.
FIG. 13 shows a micromirror which realizes such chopping-wave drive. In FIG. 13, an optical deflector 12 is comprised of oscillators 14 and 16, resilient supporting members 18 and 20, driving members 23 and 50, detecting elements 15 and 32, and a control circuit 30. This micromirror has a fundamental resonance frequency and a resonance frequency approximately three-fold the fundamental frequency, and it is driven at a combined frequency of the fundamental frequency and the three-fold frequency. Based on this, the oscillator 14 having a mirror surface is driven by a chopping-wave drive, such that optical deflection with a smaller angular speed change in the displacement angle is accomplished as compared with the sinusoidal drive.
During the driving, the oscillation of the oscillator 14 is detected by the detecting elements 15 and 32, and a driving signal necessary for the chopping wave is generated by the control circuit 30. The driving signal is inputted to the driving members 23 and 50, by which the micromirror is driven. As described above, since the angular speed of the scanning deflection has an approximately-constant angular-speed region which is widened as compared with a case where the displacement angle is based on a sinusoidal wave, the available region relative to the whole area of scanning deflection is enlarged. Here, the drive is performed in accordance with the fundamental frequency and a frequency three-fold the fundamental frequency or, alternatively, a driving frequency based on a three-fold frequency and a one-third frequency of that.